This invention relates to a diesel engine fuel control system. Specifically, this invention is directed toward a system for limiting the fuel quantity supplied to a diesel engine.
It is well known to employ exhaust gas recirculation and maximum fuel limiting to inhibit certain emissions from a diesel engine. For example, it is known to limit the fuel injected into the engine in order to prevent the air-to-fuel ratio of the cylinder charge from decreasing below a rich limit where unacceptable smoke emissions result. In general, the fuel limit must be altitude compensated since the cylinder gas charge density varies with atmospheric pressure resulting in a decrease in the air-to-fuel ratio with increasing altitude levels. This is typically accomplished by monitoring the barometric pressure and adjusting the maximum fuel limit in accord with the sensed pressure to provide for altitude compensation.
The recirculation of exhaust gases is commonly employed to inhibit the formation and emission of oxides of nitrogen. Generally, the greater the amount of exhaust gases recirculated, the lower the emission levels of oxides of nitrogen. However, since recirculated exhaust gases displace air that would otherwise be drawn into the cylinders of the diesel engine, the air-to-fuel ratio of the mixture in the cylinders is decreased with increasing levels of exhaust gas recirculation. Therefore, in order to prevent undesirable smoke emissions, the amount of exhaust gases recirculated must be limited to levels that do not result in excessively rich air-to-fuel ratios which produce smoke emissions.
In Applicants' copending application Ser. No. 342,729, filed on Jan. 26, 1982, now U.S. Pat. No. 4,399,799, an exhaust gas recirculation (EGR) control system is described wherein the amount of recirculated exhaust gases is controlled by generating a signal representing a commanded absolute pressure that is proportional to the operator commanded fuel per engine revolution. A pressure modulator generates an EGR control pressure that is equal to the commanded absolute pressure by proportionately mixing air from a subatmospheric source and atmospheric air. An EGR valve is positioned based on the difference between the EGR control pressure and atmospheric pressure. In this form of exhaust gas recirculation control system, the amount of exhaust gases recirculated is automatically compensated for atmospheric pressure changes.